1. Field of the Invention
The present invention relates to a flow measuring device, and, for example, to a device used to measure an intake-air flow rate and an intake-air temperature of an internal combustion engine.
2. Description of the Related Art
A flow measuring device in the related art is configured to have a measurement passage to let in a part of intake air flowing through a main passage, so that a flow rate detection element measuring a flow rate is installed in the measurement passage and further an intake-air temperature detection element measuring an intake-air temperature is installed in the measurement passage or the main passage.
A semiconductor device having a flow rate detection portion formed of a thin film with a thickness of a micron order is used as the flow rate detection element and a thermistor is used as the intake-air temperature detection element.
A support member is normally provided for installation of the intake-air temperature detection element. However, heat transmitted from the support member has an influence on detection accuracy of the intake-air temperature. Also, when a support member with low heat conductivity is used, a cost reduction becomes difficult.
To overcome such an inconvenience, a structure as follows is proposed as a flow measuring device capable of enhancing measurement accuracy of an intake-air temperature and reducing costs at the same time, and further achieving high robustness and high reliability.
The flow measuring device in the related art reduces influences of heat transmitted from the support member to the detection element by installing the intake-air temperature detection element in a bent measurement passage and thereby separating a connector terminal and a support terminal (see, for example, Patent Document 1).
Also, the flow measuring device in the related art reduces costs by installing the flow rate detection element and the intake-air temperature detection element on a single substrate and integrating the both into one piece.
Further, in the case of a detection element having the flow rate detection element and the intake-air temperature detection element formed on a single substrate, a speed of response to an intake-air temperature is enhanced by forming the intake-air temperature detection element on a diaphragm and reducing heat capacities of the intake-air temperature detection element and the support member including a substrate portion on which the intake-air temperature detection element is installed (see, for example, Patent Document 2).
Further, in the flow measuring device in the related art, a hole connecting the measurement passage and the main passage is provided to a cover portion near downstream of the detection element installed at a position on a plane same as one wall surface that defines the measurement passage (see, for example, Patent Document 3).
In addition, the flow measuring device in the related art has a hole provided upstream of the flow rate detection element installed in the measurement passage so as to penetrate through the measurement passage and the main passage. Hence, adhesion of water to the flow rate detection element is prevented by discharging water penetrating into the measurement passage to the main passage side (see, for example, Patent Document 4).
[Patent Document 1] JP-A-2001-124606
[Patent Document 2] JP-A-2006-138688
[Patent Document 3] Japanese Patent No. 4161077
[Patent Document 4] Japanese Patent No. 3782669
As has been described above, the structure to enhance detection accuracy of the detection element that detects an intake-air temperature is proposed for the flow measuring device in the related art. According to the techniques in Patent Documents 1 through 4, however, a heat capacity in the measurement passage in which to install the intake-air temperature detection element is so large that a response of a temperature detected in the measurement passage lags behind a response of the temperature in the main passage. Hence, the related art has a problem that even when measurement accuracy of the intake-air temperature detection element itself is enhanced, desired detection accuracy of the intake-air temperature cannot be obtained.